(1) Field of the Invention
This invention relates to methods for locating a magnetic implant, and more specifically, to a method for locating a magnetic object being guided, in a surgical application, by the field of a source magnet.
(2) Description of Related Art
In the field of surgery, there exists a need to control the orientation, forces, and/or motion of internally implanted devices. One method that has been used to control such implanted devices is the application of a magnetic field from an external magnet. In this method, the magnetic field acts upon the implanted device, which itself comprises magnetic material, which may be in the form of a permanent magnet. In accordance with prior art practice, a physician surgically implants the device comprising magnetic material and then guides the position of the implanted device by moving an external permanent magnet and observing the resultant movement directly with an X-ray fluoroscope. Examples of the prior art may be found in a review article by Gillies et al., “Magnetic Manipulation Instrumentation for Medical Physics Research,” Rev. Sci. Instrum. 65, 533 (1994). See also McNeil et al., “Functional Design Features and Initial Performance Characteristics of a Magnetic-Implant Guidance System for Stereotactic Neurosurgery,” IEEE Trans. Biomed. Engrg., 42, 793 (1995); Tillander, “Magnetic Guidance of a Catheter with Articulated Steel Tip,” Acta Radiologa 35. 62 (1951); Frei et al, “The POD (Para-Operational Device) and its Applications,” Med. Res. Eng. 5, 11 (1966); U.S. Pat. No. 3,358,676 to Frei et al., issued Dec. 19, 1967, entitled “Magnetic Propulsion of Diagnostic or Therapeutic Elements Through the Body Ducts of Animal or Human Patients”; Hilal et al., “Magnetically Guided Devices for Vascular Exploration and Treatment,” Radiology 113, 529 (1974); Yodh, et al., “A New Magnet System for Intravascular Navigation,” Med. & Biol. Engrg., 6, 143 (1968); Montgomery et al., “Superconducting Magnet System for Intravascular Navigation,” Jour. Appl. Phys. 40, 2129 (1969); U.S. Pat. No. 3,674,014 to Tillander, issued Jul. 4, 1972, entitled “Magnetically Guidable Catheter-Tip and Method”; and U.S. Pat. No. 3,794,041 to Frei et al., issued Feb. 26, 1974, entitled “Gastrointestinal Catheter.” The full content of each of the cited documents are herein incorporated by reference in their entirety.
Obviously, the above-described technique requires the physician to react to the movement of the implanted device. Determination of this movement can be a problem, because the implanted device can, in general, move in three-dimensional space inside the patient. With prior art hand-held magnets, the only feedback the surgeon could have was his observation of motion of a magnetic implant by x-ray or ultrasonic imaging in response to his movement of the magnet. Usually, fluoroscopic imaging is employed. However, fluoroscopic imaging can be subject to interference from the magnet, itself. In difficult interference situations, it is difficult without proper imaging guidance to provide even a reasonable guess as to a correct direction for the magnet axis to obtain field alignment with the intended path. The large electromagnet of Yodh et al. (supra) is one attempt to minimize the “blindness” of the approach just described, but the Yodh et al. approach still relies on operator judgment and vision, and is subject to such error. While multiple coil arrangements such as the magnetic stereotaxis system (MSS) described in McNeil et al. (supra) can be used to provide such guidance, it is difficult in such systems to provide a combined guiding force and force-applying field gradient in the same desired direction.
U.S. Pat. No. 5,558,091 issued Sep. 24, 1996 to Acker et al.,which is hereby incorporated by reference in its entirety, discloses a magnetic position and orientation determining system using magnetic fields. By monitoring field components detected at a probe during application of the fields, the position and orientation of the probe in the field can be determined. A representation of the probe can be superimposed on a separately acquired image of the subject to show the position and orientation of the probe with respect to the subject. Although the devices and methods disclosed in this patent can determine the location of an implant, the magnetic fields used are so small as to not exert any significant, perceptible forces on magnetic materials in the sensing region. There is no disclosure or suggestion to use a magnetic field to both align and/or guide an implanted probe as well as determine its location via an externally applied magnetic field, nor is there any suggestion to place strongly magnetizable materials or permanent magnets in a seed or on a probe, adjacent to a magnetic sensor, in such a manner as to allow accurate determination of location using the magnetic sensor in the immediate vicinity of the seed.
Clearly, both operation time and risk to a patient could be reduced if an apparatus and method were available to more accurately and reliably locate, as well as guide, orient, and/or move a magnetic surgical implant. (For present purposes, when reference is made to “guiding” an implant, it should be assumed that this may also refer to “orienting” an implant, as well.) Preferably, while such apparatuses and methods may allow the use of x-ray, ultrasonic, or fluoroscopic imaging devices, they should not require such imaging devices to provide the location of the implant. In addition, it would be advantageous if the location method would not require the addition of magnetic field creating devices, such as is, required by the Acker et al. patent, and which might further increase the interference with the location and operation of a guiding magnet. If the location can be obtained without the use of such additional imaging and/or locating devices, the external magnet or electromagnet (or magnets or electromagnets) used for guiding the magnetic implant may be provided with a larger, unobstructed range of motion. It would also be advantageous if the location can be obtained without being subject to interference from the magnet itself, as occurs or can occur with many common fluoroscopic imaging systems.